Conventional cranes require massive counterweights assemblies for operation, whether they are of the jib crane or derrick crane variety. Counterweights are not only an added expense of manufacture but also impart unnecessary cost in operation of mobile, vehicular crane systems when moving from one site to another. In addition, added structure is required just to support the load of counterweights whether or not the crane is in use.
Related prior art disclosures discussed in chronological order include prior U.S. Pat. Nos. 1,704,882 issued to P. E. Countryman; 1,871,603 issued to C. D. Ginter; 2,502,108 issued to W. Taylor; and 3,743,049 issued to T. Levrini.
The Countryman patent shows a crane having a boom pivotally supported at about its middle, the angular attitude of the boom being controlled by a hand wheel and screw arrangement at the rear of the boom. Thus, the rear of the boom is lowered in a controlled manner to increase the lift capacity of the crane. However, the mast of the boom are so related as to cause lift forces at the rear of the base when a load is supported and thus conventional counterweighting is mandatory.
The Ginter and Taylor patents also disclose booms pivotally mounted somewhere in their intermediate portions with the disposition of the rear of the boom being controlled so as to increase or decrease lift capacity as required. In the Ginter disclosed structure, the lift cable also controls erection of the derrick boom. However, in the case of both disclosures, load created lift forces are still transmitted to the rear of the crane so that counterbalancing is required to stabilize the crane.
The dual mast or boom structure illustrated in the Levrini patent solves its balancing problem by providing a plurality of counterweights and controls for positioning them, rather than eliminating such structure.
In direct contradistinction to the prior art as specifically disclosed in the foregoing patent disclosure, the present invention provides a crane having uniquely configured and related boom structure wherein lift forces created by load are transmitted to the front of the crane, rather than to the rear where they would have to be counterbalanced. Accordingly, a crane structure is provided capable of supporting a load far in excess of the weight of the crane itself.